Generally, Optical Parametric Chirped Pulse Amplification (OPCPA) is used to amplify an ultrashort mode-locked laser source existing in a band ranging from several femtoseconds (fs; 10−15 seconds) to several hundreds of femtoseconds. OPCPA is a new optical amplification technique of combining conventional Chirped Pulse Amplification (CPA) technology with the concept of Optical Parametric Amplification (OPA), and is laser amplification technology which has recently been actively researched.
In a conventional OPCPA apparatus, an antiparallel diffraction grating structure of applying long-wavelength preceding-type chirping (positive chirping by the antiparallelism of a grating pair) is first applied to an optical pulse stretcher, and a parallel diffraction grating structure of applying short-wavelength preceding-type chirping after amplification has been performed (negative chirping by the parallelism of a grating pair) is applied to an optical pulse compressor, so that the temporal stretch of a pulse occurring in the optical pulse stretcher is compensated for.
Such an OPCPA apparatus is described in detail with reference to FIGS. 1 to 4.
FIG. 1a is a simple diagram of a conventional OPCPA apparatus, and FIG. 1b is a detailed diagram showing the construction of the OPCPA apparatus of FIG. 1a. 
Referring to FIGS. 1a and 1b, the conventional OPCPA apparatus includes an optical pulse stretcher 10, a pump-injection dichroic mirror 80, a pump laser 20, an Optical Parametric Amplification (OPA) unit 30, a pump-removal dichroic mirror 40, a beam dumper 50, and an optical pulse compressor 60.
The optical pulse stretcher 10 temporally stretches laser light by varying the optical path of the laser light for each frequency. That is, the optical pulse stretcher 10 stretches the pulse length (pulse duration) of the output light of an ultrashort laser from an original band of several femtoseconds (fs; 10−15 seconds)/several tens of picoseconds (ps; 10−12 seconds) to a stretch band of several hundreds of picoseconds (ps; 10−12 seconds)/several nanoseconds (ns; 10−9 seconds) (in relation to this technology, refer to relevant CPA technology devised to realize efficient optical amplification and to avoid damage to optical parts).
The optical pulse stretcher used in this case and provided in the previous stage of the amplification stage applies dispersion according to wavelength, and stretches the temporal length of a pulse (temporal pulse duration) as a result of the spatial dispersion (optical path length difference), which is referred to as pulse chirping.
In the present specification, the output light of the optical pulse stretcher 10 is briefly referred to as a ‘signal’.
In the conventional OPCPA apparatus, light passes through the above optical pulse stretcher, and thus a temporally chirped signal (long wavelength precedes short wavelength) is output, as shown in FIG. 4.
The pump laser 20 is a device for supplying pump laser light (also briefly referred to as a ‘pump’).
The pump-injection dichroic mirror 80 is a device for receiving the pump and the signal, and transmitting the pump and the signal to a subsequent stage (OPA unit).
The OPA unit 30 amplifies the signal using the pump, and generates an idler. Accordingly, the pump itself is attenuated to a corresponding degree by energy conservation.
Then, the output light of the OPA unit includes the pump, the amplified signal and the idler.
Optical parametric amplification is classified into collinear phase matching and noncollinear phase matching according to the phase matching configuration between the pump and the signal. When a design condition is selected properly in the noncollinear phase matching configuration, a gain bandwidth broader than that of the collinear phase matching can be obtained. Accordingly, at the time of performing broadband Optical Parametric Amplification (broadband OPA), noncollinear phase matching is generally used. In this case, since it is difficult to subsequently use an idler due to angular dispersion according to wavelength, the idler is removed using a beam dumper.
The pump-removal dichroic mirror 40 separates the output light of the OPA unit 30 into a signal and remaining light (idler and pump), and varies the optical paths thereof.
For example, the pump-removal dichroic mirror 40 reflects the signal, passes the idler and the pump therethrough, and removes the passed idler and pump using the beam dumper 50.
Generally, since the output light, emitted from the ultrashort laser oscillator itself, has a very small amount of energy per pulse, the output light is amplified through several stages of amplification (OPA) means.
In this case, the amplification means includes pump lasers 21 and 22, OPA units 31 and 32, pump-removal dichroic mirrors 41 and 42, and beam dumpers 51 and 52. The amplification means is provided in a plural number, and thus a signal having a desired intensity can be obtained.
As described above, when the amplification of the signal to the desired intensity is performed, temporal compression is finally performed using the optical pulse compressor 60.
Reference numerals 71 to 74 are beam path changing mirrors for changing the paths of light (beam).
FIGS. 2a to 2d are diagrams showing the construction of the optical pulse stretcher of the conventional OPCPA apparatus, FIG. 2a illustrating an antiparallel diffraction grating structure (refraction type), FIG. 2b illustrating an antiparallel diffraction grating structure (reflection type), and FIGS. 2c and 2d illustrating the plan view and side view of an antiparallel diffraction grating structure (Offner—triplet type).
First, referring to FIG. 2a, the refraction-type diffraction grating structure includes two diffraction gratings (respectively designated as ‘first diffraction grating’ and ‘second diffraction grating’) 111 and 112, two lenses 113 and 114, and a single roof mirror 115.
The roof mirror 115 functions to reflect incident light at a changed height.
The optical path thereof is described. After light is incident on and diffracted from the first diffraction grating 111, the diffracted light passes through the two lenses 113 and 114 and is incident on and diffracted from the second diffraction grating 112. The diffracted light is incident on the roof mirror 115, and is reflected from the roof mirror 115 at a changed height. The reflected light is incident on the beam path changing mirror 71 through the second diffraction grating 112, the two lenses 114 and 113, and the first diffraction grating 111.
In this case, a corresponding separation distance (that is, a corresponding separation distance between gratings which has same chirping power, in parallel grating structure) is represented by 2f−s1−s2, where f denotes the focal distance of the lenses 113 and 114, and s1 and s2 denote the distances between the lens 113 and the diffraction grating 111 and between the lens 114 and the diffraction grating 112, respectively.
In the refraction-type antiparallel diffraction grating structure of FIG. 2a, the following problems may occur. That is, in the refraction-type antiparallel diffraction grating structure including the lenses 113 and 114, chromatic aberration caused by the lenses, etc. may occur.
In order to solve the problem of chromatic aberration caused by the lenses, the reflection-type antiparallel diffraction grating structure of FIG. 2b has been devised.
Referring to FIG. 2b, the reflection-type antiparallel diffraction grating structure includes two diffraction gratings (respectively designated as ‘first diffraction grating’ and ‘second diffraction grating’) 121 and 122, two cylinder mirrors 123 and 124, and a single roof prism 125.
The roof prism 125 performs the same function as the roof mirror of FIG. 2a. 
The optical path thereof is described. After light is incident on and diffracted from the first diffraction grating 121, the diffracted light is incident on and diffracted from the second diffraction grating 122 through the two cylinder mirrors 123 and 124. The diffracted light is incident on the roof prism 125. The incident light is reflected from the roof prism 125 at a changed height, and the reflected light is incident on the beam path changing mirror 71 through the second diffraction grating 122, the cylinder mirrors 124 and 123, and the first diffraction grating 121.
In this case, a corresponding separation distance is represented by 2f−s1−s2, where f denotes the focal distance of the cylinder mirrors 123 and 124, and s1 and s2 denote the distances between the cylinder mirror 123 and the diffraction grating 121, and between the cylinder mirror 124 and the diffraction grating 122.
The reflection-type antiparallel diffraction grating structure of FIG. 2b may have the following problem. Specifically, a problem of aberration (such as astigmatism and coma) caused by the incline of the two cylinder mirrors relative to the optical axis occurs.
In order to solve the above problem, an Offner-triplet structure of FIG. 2c (plan view) and FIG. 2d (side view) has been devised.
Referring to FIGS. 2c and 2d, the Offner-triplet structure includes a single diffraction grating 131, two spherical mirrors (respectively designated as ‘first spherical mirror’ and ‘second spherical mirror’) 132 and 133 having different sizes, and a single roof prism 134.
In the above construction, the roof prism 134 performs the same function as the roof mirror of FIG. 2a. 
The second spherical mirror 133 is larger than the first spherical mirror 132.
The optical path of the Offner-triplet structure is described. After light is incident on and diffracted from the diffraction grating 131, the diffracted light is incident on and reflected from the second spherical mirror 133. After the reflected light is incident on and reflected from the first spherical mirror 132, the reflected light is incident on and reflected from the second spherical mirror 133 again. The reflected light is incident on and diffracted from the diffraction grating 131 again, and the diffracted light is incident on the roof prism 134. The incident light is reflected from the roof prism 134 at a changed height. The reflected light passes through the diffraction grating 131, the second spherical mirror 133, and the first spherical mirror 132, and is then output through the second spherical mirror 133 and the diffraction grating 131.
In this case, a corresponding separation distance is represented by 2(R−s), where R denotes the radius of curvature of the second spherical mirror 133, and s denotes the distance from the second spherical mirror 133 to the diffraction grating 131.
FIG. 2e is a graph showing signal chirping performed through the antiparallel diffraction grating structure in the conventional optical pulse stretcher.
FIG. 2e illustrates the results of computer simulation of the signal chirping generated by a typical optical pulse stretcher. In the simulation with the Offner-triplet structure of FIGS. 2c and 2d, parameters for the diffraction gratings are set as follows. That is, first-order diffraction is used, the number of grooves of the diffraction grating is set to 1740 lines/mm, the angle of incidence is set to 62.8°, the angle of diffraction is set to 70.8°, and a corresponding separation distance is set to 530 mm.
As a result, the signal having an 8 nm spectrum ranging from 1050 nm to 1058 nm) centered on a wavelength of 1054 nm is stretched by about 800 ps. Such a value (800 ps) is the value obtained by dividing an optical path-length difference by the speed of light, where the optical path-length difference is 240 mm, and the speed of light is three hundred thousand km/s.
For reference, laser light having passed through the optical pulse stretcher has a pulse structure which is temporally stretched according to wavelength. In this case, the reason an optical pulse is temporally stretched according to wavelength is described below. That is, ultrashort laser light includes laser light having different wavelength components. Accordingly, as light passes through the diffraction grating structure for respective wavelength components, the light travel distance thereof varies, and consequently an optical pulse is temporally stretched due to the difference between the temporal delays of respective wavelength components, attributable to the variation in light travel distance.
FIG. 3a is a diagram showing the construction of the optical pulse compressor of the conventional OPCPA apparatus.
Referring to FIG. 3a, the parallel diffraction grating structure includes two diffraction gratings 141 and 142 having a parallel arrangement, and a single roof mirror 143 for reflecting incident light at a changed height.
The optical path thereof is described below. After light is incident on and diffracted from the first diffraction grating 141, the diffracted light is incident on and diffracted from the second diffraction grating 142. The diffracted light is incident on the roof mirror 143, and is reflected from the roof mirror 143 at a changed height. The reflected light is output through the second diffraction grating 142 and the first diffraction grating 141.
In this case, the corresponding separation distance can be represented by the distance between the two diffraction gratings 141 and 142.
FIG. 3b is a graph showing signal chirping performed by a parallel diffraction grating structure in the conventional optical pulse compressor.
FIG. 3b illustrates the results of computer simulation of the signal chirping structure generated in a typical optical pulse compressor. In the parallel diffraction grating structure of FIG. 3b, parameters for diffraction gratings are set as follows. That is, −1st-order diffraction is used, the number of grooves of each diffraction grating is set to 850 lines/mm, the angle of incidence is set to 5.0°, the angle of diffraction is set to 79.4°, and a corresponding separation distance is set to 692 mm.
As a result, a signal having an 8 nm spectrum centered on a wavelength of 1054 nm is temporally stretched by about 800 ps. In this case, the optical path-length difference is about 240 mm.
Further, such an optical pulse compressor can temporally compress the signal, which has an 8 nm spectrum centered on a wavelength of 1054 nm and which has been temporally stretched by a typical optical pulse stretcher by about 800 ps, to the original state thereof.
Next, the operation and effects of the conventional OPCPA apparatus are described below.
First, an original signal passes through the optical pulse stretcher 10 having the construction of FIGS. 2a to 2d, so that a temporally stretched waveform, in which a long wavelength precedes a short wavelength, is output (refer to FIG. 4).
The output light (signal) of the optical pulse stretcher 10 is incident on a first pump-injection dichroic mirror 81 through the beam path changing mirrors 71 and 72, and the pump light output from the first pump laser 21 is also incident on the first pump-injection dichroic mirror 81.
Both the signal and pump are incident on the first OPA unit 31. In the first OPA unit 31, as the signal is amplified using the pump, an idler is generated, and the pump itself is attenuated.
Consequently, the output light of the first OPA unit 31 includes the pump, the amplified signal, and the idler.
The output light is incident on the first pump-removal dichroic mirror 41, and is separated into the amplified signal and remaining light (pump and idler). That is, both the attenuated pump and the idler pass through the first pump-removal dichroic mirror 41 and are removed by the first beam dumper 51, and the amplified signal is reflected from the first pump-removal dichroic mirror 41.
If the amplified signal succeeds in reaching the level of aimed intensity or more, the amplified signal is incident on the optical pulse compressor 60, otherwise, it undergoes the above process again (process from the pump-injection dichroic mirror to the beam dumper).
That is, if the signal (amplified signal) reflected from the first pump-removal dichroic mirror 41 fails in reaching the level of aimed intensity or more, the signal is incident on the other pump-injection dichroic mirror (second pump-injection dichroic mirror) 82, and the pump generated from the second pump laser 22 is also incident on the second pump-injection dichroic mirror 82. Thereafter, the signal and the pump pass through the second OPA unit 32, the second pump-removal dichroic mirror 42, and the second beam dumper 52. The above process is repeated until the above signal is amplified to a predetermined intensity or more.
Finally, the amplified signal is incident on the optical pulse compressor 60, and the optical pulse compressor 60 temporally compresses the amplified signal.
However, the conventional OPCPA apparatus has the following problems.
That is, the conventional OPCPA apparatus forms a linear pulse chirping structure using second-order dispersion (Group Velocity Dispersion: GVD). However, such a structure is problematic in that, since the second-order dispersion terms in the signal and idler, which are the output light of the OPA unit, have the same magnitude and have opposite signs, compensation for pulse chirping is difficult in the optical pulse compressor. Therefore, an idler having energy intensity roughly equal to that of the amplified signal cannot be generally utilized, and must be discarded.
For a detailed description of dispersion, reference may be made to FIG. 7.